Secondary emission structure

ABSTRACT

A grid is coated with a dielectric material which has good secondary emission characteristics and the coated grid is mounted in a cathode ray tube along with a collector electrode. The secondary emission structure is used as an expansion lens or a storage mesh in a storage type cathode ray tube.

United States Patent [191 Balint et al.

[ Nov. 13, 1973 SECONDARY EMISSION STRUCTURE Inventors: Francis A. Balint, Colorado Springs,

Colo.; William P; Kruger, Los Altos, Calif.; Milton E. Russell, Colorado Springs; Lucien F. Trueb, Denver, both of Colo.

Assignee: Hewlett-Packard Company, Palo Alto, Calif.

Filed: June 19, 1972 Appl. No.: 263,834

Related US. Application Data Continuation of Ser. No. 75,809, Sept. 28, 1970, abandoned.

US. Cl 313/68 R rm. Cl. l-10lj 31/18 Field of Search 313/68 R US ELECTRON BEAM ,J

GUN DEFLECTORS [56] References Cited UNITED STATES PATENTS 2,169,046 8/1939 Headrick 313/68 R 2,203,048 6/1940 Farnsworth et al.. 313/68 R 3,005,125 10/1961 Evans et al 313/92 Bl 3,250,942 5/1966 Yoshida et al.... 313/297 3,331,983 7/1967 Hesse 315/12 OTHER PUBLlCATlONS Chemical Abstracts, Vol. 70, 1969, pg. 282

Primary Examiner-Herman Karl Saalbach Attorney-Hewlett Packard [57] ABSTRACT 3 Claims, 2 Drawing Figures STORAGE CIRCUITS PAIENIEDunvmszs 3.772.553

ELECTRON GUN m N) .L J

INVENTORS FRANClS A. BALlNT WILLIAM P. KRUGER MILTON E. RUSSELL LUCIEN F. TRUEB SECONDARY EMISSION STRUCTURE This is a continuation of US. Pat. application Ser. No. 75,809, filed Sept. 28, 1970 now abandoned.

BACKGROUND OF THE INVENTION The expansion lens in a cathode ray tube (CRT) is usually a metal grid at the electron gun end of the post accelerator region. When the high energy electron beam from the electron gun passes through the expansion lens grid, secondary electrons are often knocked off. These electrons are pulled along with the electron beam and they form a halo or second spot on the CRT display screen near the trace made by the electron beam from the electron gun. In the prior art, the usual solution to this problem was to select a grid material with low secondary emission characteristics. (See Handbook of Materials & Techniques for Vacuum Devices, Walter H. Kohl, Reinhold Publishing Company, New York, 1967.)

The storage mesh in a storage or variable persistence type CRT is grid coated on one side with a material optimized for its secondary emission characteristics. Materials commonly used for such a coating include magnesium fluoride, zinc sulfide, and magnesium oxide. A major problem with such coatings is that they dissociate under continued exposure to the high energy electron beam. The result of this dissociation is a bright or burned spot on the display screen of the CRT.

SUMMARY OF THE INVENTION If the expansion lens grid is coated on the electron gun side with a dielectric coating which emits secondary electrons and a collector electrode is placed between the electron gun and the grid, then the halo effect can be eliminated. The electron beam will knock secondary electrons off the coating on the grid, and there will then be a net positive charge at that location. The secondary electrons will be captured either by the collector electrode or the positively charged areas on the expansion lens, and they will not pass through the expansion lens into the post accelerator region. Thus there will be no halo because substantially no secondary electrons reach the display screen.

It has been found that the burning of a storage mesh in a storage or variable persistence CRT occurs because the dielectric coating is dissociated by the electron beam if the dielectric coating lattice energy is less than the energy of the electron beam. The lattice energy of the dielectric coating is determined by the refined Kapustinsky method. It has consequently been found that if the lattice energy of the coating is greater than 1,000 kilocalories per mole (kcal/mole), the energy in electron beams used in CRTs is not enough to cause the material to dissociate. Thorium oxide and hafnium dioxide, for example, are dielectrics with lattice energies higher than 1,000 kcal/mole and such materials are suitable for use on expansion lenses and storage meshes.

DESCRIPTION OF THE PREFERRED EMBODIMENT A secondary emission structure 10 shown in FIG. 1 comprises a grid 12 supported by a support 14 with a dielectric coating 16 on one surface 20 of grid 12. A

collector electrode 18 is adjacent to and spaced apart from surface 20 of grid 12. Collector electrode 18 may take different shapes, depending upon the application of secondary emission structure 10. In FIG. 1 collector electrode 18 is shown as a metal annulus, though it can also be a grid.

A cathode ray tube 30 shown in FIG. 2 comprises a glass envelope 32, an electron gun 34, beam deflectors 36, a flood gun 37, an expansion lens 38, a storage mesh 40, and a display screen 42. An electron beam 44 is shown emanating from electron gun 34, passing through beam deflectors 36, and expansion lens 38 and impinging on storage mesh 40 and display screen 42 forming a trace 46 on display screen 42. A collector ring 39 is disposed on the electron gun side of expansion lens 38 and is maintained at a positive potential of approximately 50 volts with respect to expansion lens 38 by battery 48. A collector mesh 41 is disposed on the electron gun side of storage mesh 40. Display screen 42 has a transparent conductive coating 43, and coating 43, storage mesh 40 and collector mesh 41 are connected to storage circuits 50 which control the potentials on elements 40, 41, and 43.

When electron beam 44 passes through expansion lens 38, secondary electrons may be knocked off dielectric coating 16. Since dielectric coating 16 is only on the electron gun side of expansion lens 38, the secondary electrons will scatter toward collector ring 39 and most secondary electrons will be captured by collector ring 39. The portion of dielectric coating 16 struck by electron beam 44 will be charged positive and will recapture substantially all secondary electrons not captured by collector ring 39. Since substantially no secondary electrons are carried along with electron beam 44, no halo effect will appear on display screen 42 near trace 46.

When electron beam 44 passes through storage mesh 40, it will knock off some secondary electrons which will be collected by collector mesh 41. A net positive charge will remain on storage mesh 40 where the secondary electrons were knocked off. This charge pattern constitutes a stored trace. Flood gun 37 emits a flood of low energy electrons toward storage mesh 40 and the electrons will pass through the mesh only at those points which have a net positive charge; the remainder of the storage mesh has a net negative charge which will repel the flood electrons. The flood electrons which pass through storage mesh 40 make a trace on display screen 42 which duplicates the stored trace on storage mesh 40. If electron beam 44 remains directed at one spot on storage mesh 40 for more than 5 or 10 minutes, enough of the beam energy will usually be transferred to dielectric coating 16 to cause some of the dielectric material to dissociate, if the lattice energy of the material is lower than the energy of electron beam 44. With presently used materials such as magnesium fluoride, magnesium oxide and zinc sulfide, the lattice energy is less than commonly used electron beam energies. Dissociation of the dielectric material causes burned spots on storage mesh 40 which will no longer store a negative charge. These spots allow flood electrons to pass through, causing bright spots on display screen 42. If materials such as hafnium dioxide or thorium oxide are used as dielectric coating 16, there will be no burn effect since the lattice energies of these materials are 2,740 kcal/mole and 2,510 kcallmole respectively, and CRT electron beam energies are well below that level. Such dielectric materials with lattice energies above 1,000 kcal/mole are appropriate for use on expansion lens 38 as well as storage mesh 40,

since the dielectric coating 16 on expansion lens 38 should not dissociate when exposed to the electron beam.

in practice, a CRT may have an expansion lens and a storage mesh, or it may only have one; and CRT 30 in FIG. 2 includes both only for illustrative purposes.

We claim:

1. In a cathode ray tube including an electron gun disposed at one end of the tube for emitting an electron beam, a display screen disposed at an opposite end of the tube for receiving the electron beam, electron deflectors for deflecting the electron beam disposed adjacent the electron gun, and a post accelerator region between the electron deflectors and the display screen, an expansion lens disposed adjacent the electron deflectors at the incident end of the post accelerator region comprising:

a grid having a surface facing the electron gun;

a dielectric coating on said surface for enhancing secondary electron emission from said surface in response to the electron beam impinging thereon, said impingement forming charged areas on the dielectric coating; and a collector electrode disposed adjacent to and insulated from said surface for collecting secondary electrons, whereby secondary electrons from said surface are kept from being carried along with the electron beam by being collected by the collector electrode and by the charged areas on the dielectric coating. 2. A cathode ray tube as in claim 1 wherein the dielectric coating has a unit lattice energy greater than the equivalent unit energy of the electron beam.

3. A cathode ray tube as in claim 2 wherein the dielectric coating has a lattice energy greater than 1,000

kcal/mole. 

1. In a cathode ray tube including an electron gun disposed at one end of the tube for emitting an electron beam, a display screen disposed at an opposite end of the tube for receiving the electron beam, electron deflectors for deflecting the electron beam disposed adjacent the electron gun, and a post accelerator region between the electron deflectors and the display screen, an expansion lens disposed adjacent the electron deflectors at the incident end of the post accelerator region comprising: a grid having a surface facing the electron gun; a dielectric coating on said surface for enhancing secondary electron emission from said surface in response to the electron beam impinging thereon, said impingement forming charged areas on the dielectric coating; and a collector electrode disposed adjacent to and insulated from said surface for collecting secondary electrons, whereby secondary electrons from said surface are kept from being carried along with the electron beam by being collected by the collector electrode and by the charged areas on the dielectric coating.
 2. A cathode ray tube as in claim 1 wherein the dielectric coating has a unit lattice energy greater than the equivalent unit energy of the electron beam.
 3. A cathode ray tube as in claim 2 wherein the dielectric coating has a lattice energy greater than 1,000 kcal/mole. 